Mediterranean Slave Cargoes and Markets
The Christian Church of the time was well-aware of the evils of the Mohammedan slave trade, but throughout the Middle Ages the slave trade was a recognized institution, and it did denounce certain forms of the slave traffic, prohibited Jews from dealing in Christian slaves and condemned the sale of Christians to Moslems.  Charlemagne wrote Pope Hadrian that Rome merchants were reportedly selling slaves to the “unspeakable Saracens,” to which Hadrian replied: “Never have we fallen into such wickedness, nor has any such deed been done with our permission.”
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Mediterranean Slave Cargoes and Markets:
“The Moslem was not at first adapted to the life of the sea, but when he had learned the art of navigation, piracy became a most profitable method of supplying the slave market.  Piracy was more profitable because of the rich returns to be derived from payment of ransoms and the Mediterranean sea remained unsafe for Christian travel long after the forces of civilization had put an end to the slave trade on the European continent.
In the Ninth Century the Saracens were quick to take advantage of the helpless condition of Italy.  Grave fears were felt for the city of Rome.  At this time Pope John VIII wrote to Charles the Bald, “If all the trees of the forest became tongues, they could not describe the ravages of the impious pagans; the devoted people of God are destroyed by continuous slaughter; he who escapes fore and sword is taken into slavery.  Cities, castles, and villages are wasted and without a single soul; bishops wander and get their bread as beggars, and flee to Rome as the only place of refuge.”
It is interesting to note, that the Moslems were not the only people engaged in the Mohammedan slave trade, but its profits went alike to Christian, Jew and Moslem. The Jews are said to have amassed great wealth as a result of the trade. This was especially true of the Spanish Jews in the time of the Moorish occupation. The Moors were far more tolerant of Jews than the Spanish Christians had been, and the rich Spanish Jews are said to have vied with the richest Moors in extravagance and ostentation. Much of their wealth came from this form of trade.
Great profits were made in Slav captives. The name Slav was at first synonymous with slave and was applied to others than the present Slavs.
The large profits early attracted the merchants of Italy [and the]….traffic flourished in the Eighth Century.  One of the letters of Charlemagne inquires into the rumor that had reached him, that at Rome Christians were being sold to the Infidel. Moslem pirates sold slaves to the Christians and the Christians sold slaves to the Moslem.
Public slave auctions were held in Venice, Verona, Genoa, and Pisa.  Tartars, Russians, Bosnians, Slavs, Greeks, and Teutons, came into these markets.  Here slaves of Africa and Asia were imported into Europe and other slaves went on to the Barbary and Egyptian markets.
Henry Hallam says, “It is a humiliating proof of the degradation of [Christendom], that the Venetians were reduced to purchase the luxuries of Asia by supplying the slave markets of the Saracens; the apology would perhaps have been, that they were purchased from their heathen neighbors, but a slave dealer was not very inquisitive as to the faith or origin of his victim.”
Pope John XXIII accused the Genoese before the face of the world, of contributing to the power of the infidel through furnishing them slaves.  By the 13th century a very lucrative trade in slaves had been built up by the merchants of Genoa. These slaves were brought from the Black Sea region to Egypt.  Beybars, the caliph of Egypt, had established an alliance with Beraka, the Khan of the…Mongols….who pastured in the valley of the Volga.  The wars and the raid of the Khan helped to fill the market of Caffa.  The Genoese used shrewd business methods to obtain…the right to bring the slave ships through the Bosporus. These ships often carried many Christian captives to Egypt.
The experience of the captive on his journey must have been horrible, for it is estimated that only one in ten ever reached Caffa and the horror of the sea voyage, via Constantinople became so apparent, that Genoa had to enact strict sanitary laws for control of the ships.” 
(The Mohammedan Slave Trade, J.H. Johnson, Journal of Negro History, Vol. XIII, No. 2, April, 1928, Carter G. Woodson, editor, excerpt, pg. 482- 485)